Stoppers of this type to which the present invention relates is for infusion bottles are described in, for example, DE 3,241,283 C3, DE 4,135,470 A1, DE 4,228,090 C2, and DE 4,344,134 A1. All of them have a collar which is very long in comparison to the edge and also to the thickness H.sub.1 of the puncture area, with the result that the collar projects quite far into the neck of the bottle. In addition, the top surface of the puncture area is uniform in height and is level with the edge of the stopper. In these known stoppers for infusion bottles, the puncture area is relatively thick; this is done to prevent the puncture needle of the infusion kit from sliding out of the stopper under its own weight and the weight of the infusion liquid present in it, in the drip chamber, and in the tubing connected to the needle after the infusion bottle has been hung upside down. In this context, reference can be made to the standards DIN 38,363 and ISO 8,536.2, which pertain to these types of sealing stoppers, also called "hollow stoppers". In the test of the strength with which the puncture needle is held in the stopper according to the DIN standard, the test needle may not slide out of the stopper for 5 hours under an additional load of 0.5 kg. To ensure, therefore, that a sufficient amount of lateral pressure can be exerted on the inserted needle and to increase the friction between the surface of the needle and the surface of the hole formed in the puncture area of the stopper by the puncture, the puncture area of stoppers of this type is made quite thick, so that it is held relatively rigidly between the edge and the collar. The disadvantage of this stopper is that a relatively large amount of force is required to push the relatively thick puncture needle of an infusion kit through it. In this regard, furthermore, the above-cited standards, especially the DIN standard, specify that, in the test of the puncturability of the stopper, the force required to push the needle through the stopper may not exceed 100 N.
Against the background of these known stoppers for infusion bottles, it is therefore the goal of the present invention to design a stopper of the general type in question in such a way that much less effort is required to push the puncture needle of an infusion kit through the stopper, which means that, even though the puncture area of the stopper is much thinner, the infusion kit is nevertheless securely held by the stopper on the upside-down infusion bottle and thus prevented from sliding out.